https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85441
Bug ID: 85441
Summary: Empty loops not optimised away
Product: gcc
Version: 7.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: sebastian321123 at interia dot pl
Target Milestone: ---
This code compiles for a long time and has high memory usage (roughly 2GiB). I
suspect that empty loops are not removed.
Code:
constexpr double foo(const unsigned int N) {
for (unsigned int i = 0; i != N; ++i) {
for (unsigned int i = 0; i != N; ++i) {
}
}
return 7;
}
int main() {
return foo(4393u);
}
Command line:
$ time env g++ -v --std=c++14 foo.cpp -o foo
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --enable-libmpx --with-system-zlib --with-isl
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu
--disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-plugin
--enable-install-libiberty --with-linker-hash-style=gnu
--enable-gnu-indirect-function --enable-multilib --disable-werror
--enable-checking=release --enable-default-pie --enable-default-ssp
Thread model: posix
gcc version 7.3.1 20180312 (GCC)
COLLECT_GCC_OPTIONS='-v' '-std=c++14' '-o' 'foo' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/cc1plus -quiet -v -D_GNU_SOURCE foo.cpp
-quiet -dumpbase foo.cpp -mtune=generic -march=x86-64 -auxbase foo -std=c++14
-version -o /tmp/cc3hVZ3b.s
GNU C++14 (GCC) version 7.3.1 20180312 (x86_64-pc-linux-gnu)
compiled by GNU C version 7.3.1 20180312, GMP version 6.1.2, MPFR
version 4.0.1, MPC version 1.1.0, isl version isl-0.18-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
ignoring nonexistent directory
"/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../x86_64-pc-linux-gnu/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../include/c++/7.3.1
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../include/c++/7.3.1/x86_64-pc-linux-gnu
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../include/c++/7.3.1/backward
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/include
/usr/local/include
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/include-fixed
/usr/include
End of search list.
GNU C++14 (GCC) version 7.3.1 20180312 (x86_64-pc-linux-gnu)
compiled by GNU C version 7.3.1 20180312, GMP version 6.1.2, MPFR
version 4.0.1, MPC version 1.1.0, isl version isl-0.18-GMP
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: 3fb60f63f2e4dccab1b015b716b8b627
COLLECT_GCC_OPTIONS='-v' '-std=c++14' '-o' 'foo' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
as -v --64 -o /tmp/ccO93I4M.o /tmp/cc3hVZ3b.s
GNU assembler version 2.29.1 (x86_64-pc-linux-gnu) using BFD version (GNU
Binutils) 2.29.1
COMPILER_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/
LIBRARY_PATH=/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../lib/:/lib/../lib/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../:/lib/:/usr/lib/
COLLECT_GCC_OPTIONS='-v' '-std=c++14' '-o' 'foo' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/collect2 -plugin
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/liblto_plugin.so
-plugin-opt=/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/lto-wrapper
-plugin-opt=-fresolution=/tmp/cckfgL6n.res -plugin-opt=-pass-through=-lgcc_s
-plugin-opt=-pass-through=-lgcc -plugin-opt=-pass-through=-lc
-plugin-opt=-pass-through=-lgcc_s -plugin-opt=-pass-through=-lgcc --build-id
--eh-frame-hdr --hash-style=gnu -m elf_x86_64 -dynamic-linker
/lib64/ld-linux-x86-64.so.2 -pie -o foo
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../lib/Scrt1.o
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../lib/crti.o
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/crtbeginS.o
-L/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1
-L/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../lib -L/lib/../lib
-L/usr/lib/../lib -L/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../..
/tmp/ccO93I4M.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/crtendS.o
/usr/lib/gcc/x86_64-pc-linux-gnu/7.3.1/../../../../lib/crtn.o
COLLECT_GCC_OPTIONS='-v' '-std=c++14' '-o' 'foo' '-shared-libgcc'
'-mtune=generic' '-march=x86-64'
env g++ -v --std=c++14 foo.cpp -o foo 21.46s user 1.34s system 99% cpu 22.901
total
I am using Arch Linux. I can also confirm this behaviour for --std=c++17.
